\(\Rightarrow\dfrac{v2}{v1}=\dfrac{3v1}{v1}=3=\dfrac{gt2}{gt1}\)
\(\Rightarrow\dfrac{t2}{t1}=3\Leftrightarrow\dfrac{\sqrt{\dfrac{2.h2}{g}}}{\sqrt{\dfrac{2.ho}{g}}}=3\Leftrightarrow\dfrac{\dfrac{2h2}{g}}{\dfrac{2ho}{g}}=9\)
\(\Leftrightarrow\dfrac{h2}{ho}=9\Leftrightarrow h2=9ho=9.20=180m\)